The impacts of the reforestation of the Sahel-Sahara interface on the seasonal distribution of the surface temperature and thermal extremes are studied in the Sahel (West African region lying between 11 °N and 18 °N). We performed a simulation with the standard version of the RegCM4 model followed by another one using the altered version of the same model taking into account an incorporated forest. The impacts of the vegetation change are assessed by analyzing the difference between the two runs. The reforestation may influence strongly the frequency of warm days (TG90P) and very warm days (TX90P) by decreasing it over the reforested zone from March to May (MAM) and the entire Sahel during the June-August (JJA) period. These TG90P and TX90P indices decrease may be due to the strengthening of the atmospheric moisture content over the whole Sahel region and the weakening of the sensible heat flux over the reforested zone. The analysis also shows a decrease of the TN90P indice (warm nights) over the Sahel during the wet season (JJA) which could be partly associated with the strengthening of the evapotranspiration over the whole Sahel domain. The analysis of additional thermal indices shows an increase of the tropical nights over the entire Sahel from December to February (DJF) and during the warm season (MAM). The strengthening of the tropical night is partly associated with an increase of the surface net downward shortwave flux over the reforested zone. When considering the heat waves, an increase (a decrease) of these events is recorded over the southern Sahel during JJA and SON periods (over the whole Sahelian region during DJF), respectively. Changes in latent heat flux appear to be largely responsible for these extreme temperatures change. This work shows that the vegetation change may impact positively some regions like the reforested area by reducing the occurrence of thermal extremes; while other Sahel regions (eastern part of the central Sahel) could suffer from it because of the strengthening of thermal extremes.
The sustainability of economic development and living conditions depends on our ability to manage the risks associated with extreme events. Many practical problems require knowledge of the behavior of extreme values. In particular, human health is sensitive to high values of warm extreme. Moreover, a high consensus exists on the reality of global warming across the world [
It’s more difficult for developing countries particularly those of the Sahelian region known for their low income to cope with an increase of heat waves compared to developed countries of Europe where capacities in mitigation and adaptation are considered to be more efficient. For example in 2010, Niger has experienced episodes of extreme heat that it hadn’t known for the last 130 years and such thermal extremes had many impacts on agriculture and morbidity of the population [
Furthermore, the 21st century global warming due to anthropogenic forcing will be strong over Africa, which is likely to increase the number and duration and amplitude of heat waves, especially in arid and semi-arid regions of the Sahel [
Likewise, Fontaine et al. [
Understanding the heat waves characteristics and the associated physical mechanisms is highly needed to develop adequate mitigation and adaptation strategies. Studies by [
Moreover, several studies [
All these pioneering studies cited above helped to better understand the heat waves characteristics and associated mechanisms in West Africa. They also emphasized the key role of land vegetation cover changes on the variability of West African climate. However, none of these studies have attempted to investigate the impacts of the reforestation at the Sahel-Sahara interface (which is the transition region between the semi-arid region called Sahel and the Sahara desert) on the West African daily extreme temperature events for the present-day. Then the necessity to take into account the impacts of a possible change of land cover properties (especially changes in vegetation) on the occurrence of such extremes becomes an important topic to investigate. This work is oriented in this perspective. It investigates the impacts of the reforestation of the Sahel-Sahara interface on the annual distribution of surface temperature and extreme heat temperatures over the Sahel region as well as the atmospheric factors associated to this variability using the International Center for Theoretical Physics (ICTP) regional climate model RegCM4. As the reforestation activities could increase or decrease extremes events, there is a need for detailed studies on how the reforestation may alter the present climate. The overall question addressed here, is to know which Sahel sub-regions could benefit or suffer when reforesting the Sahel-Sahara interface as proposed in the Great Green Wall (GGW) project. The GGW project is an African initiative which goal is to green a strip of land extending from Senegal to Djibouti in order to fight against the desertification and climate change consequences (https://www2.unccd.int/actions/great-green-wall-initiative). The paper is structured as follows. Section 2 describes the experimental design, methods and the thermal indices used following the guidelines of the Expert Team on Climate Change Detection and Indices (ETCCDI). Section 3 presents the results and the discussion. Finally in Section 4, concluding remarks are provided.
We used the latest version of the regional climate model RegCM4.5 developed at the Abdu Salam International Center for Theoretical Physics (ICTP) as described in [
For our experiments, interactions between the land surface and the atmosphere are described using the Biosphere Atmosphere Transfer Scheme (BATS1E; [
The Land surface processes are represented by the Biosphere Atmosphere Transfer Scheme 1E (BATS1E) as described in [
boundaries and are carried out at a spatial resolution of 50 km. The simulation domain (
The observed monthly mean surface temperatures of Climate Research Unit (CRU) TS3.22 [
1) Crop/mixed farming |
---|
2) Short grass |
3) Evergreen needle leaf tree |
4) Deciduous needle leaf tree |
5) Deciduous broadleaf tree |
6) Evergreen broadleaf tree |
7) Tall grass |
8) Desert |
9) Tundra |
10) Irrigated crop |
11) Semi-desert |
12) Icecap/glacier |
13) Bog or marsh |
14) Inland water |
15) Ocean |
16) Evergreen shrub |
17) Deciduous shrub |
18) Mixed woodland |
19) Forest/field mosaic |
20) Water and land mixture |
Parameters | Short Grass | Tall Grass | Desert | Semi-Desert | Forest |
---|---|---|---|---|---|
Max fractional vegetation cover | 0.80 | 0.80 | 0.00 | 0.35 | 0.80 |
Vegetation albedo for wave lengths < 0.7 μm | 0.10 | 0.08 | 0.20 | 0.17 | 0.06 |
Vegetation albedo for wave lengths > 0.7 μm | 0.30 | 0.30 | 0.40 | 0.34 | 0.18 |
Difference between max fractional Vegetation cover and cover at 269 K | 0.1 | 0.0 | 0.2 | 0.0 | 0.4 |
Roughness length (m) | 0.05 | 0.10 | 0.05 | 0.10 | 0.30 |
Min stomatal resistence (s/m) | 60 | 60 | 200 | 150 | 120 |
Max Leaf Area Index | 2 | 6 | 0 | 6 | 6 |
Min Leaf Area Index | 0.5 | 0.5 | 0 | 0.5 | 0.5 |
Stem (dead matter area index) | 4.0 | 2.0 | 0.5 | 2.0 | 2.0 |
Root zone soil layer depth (mm) | 1000 | 1000 | 1000 | 1000 | 2000 |
Soil texture type | 6 | 6 | 3 | 5 | 6 |
Soil color type | 3 | 4 | 1 | 2 | 4 |
and driven by the ERA-Interim reanalysis (spatial resolution of 1.5 × 1.5; [
We first validated the simulations of the standard version of the RegCM4 model before studying the impacts of the reforestation on the seasonal distribution of the surface temperature and thermal extremes by comparing the mean surface temperature simulated by the model with the CRU observations data.
Index | Index abbreviate | Unit |
---|---|---|
Percentage of days with daily Tm (mean surface temperatures) > 90th percentile on running 5 day windows (= warm days) | TG90P | % |
Percentage of days with daily TX (maximum temperatures) > 90th percentile on running 5 day windows (= very warm days) | TX90P | % |
Percentage of nights with daily minimum temperatures TN >90th percentile on running 5 day windows (= warm nights) | TN90P | % |
«Hot Days: “HtD”» (= days with TX > 35˚C) | HtD | Count in day |
«Tropical Nights: “TrN”» (= nights with TN > 20˚C) | TrN | Count in day |
Number of Heat Waves | HW | Count in day |
Warm spell days | WSD | Length in day |
temperature over the Sahara desert (below 18˚C) and the maxima temperature (above 30˚C) over the Sahel. The RegCM4 model underestimated (overestimated) the surface temperature over the Sahara desert (over the Sahel), respectively during this same period. Higher values (above 32˚C) of surface temperature are observed by CRU and simulated by RegCM4 over the Sahel from March to May (MAM); the MAM period is considered as the Sahel warm season. During the summer season (June-August; JJA), the CRU observations data (
Globally, the analysis reveals that the cold biases are more visible in the desert areas and the coastlines along the Gulf of Guinea.
It is difficult to determine the causes of the Regional Climate Models (RCMs) temperature biases [
The first lines of this part are devoted to the analysis of some atmospheric parameters necessary to a better interpretation of the seasonal distribution of thermal extremes over the Sahel region (West African region lying between 11˚N and 18˚N).
small decrease of the evapotranspiration (−0.4 cg/m2/s) is recorded in the Atlantic Ocean during DJF, JJA and SON.
The sensible heat flux averaged from 2003 to 2009 for RegCM4 (top) and the difference between the two runs (down) is displayed in
The latent heat flux averaged from 2003 to 2009 for RegCM4 (top) and the difference between both runs (down) is displayed in
(>50 W∙m−2) located over the southern Sahel and the ocean and the minima (<20 W∙m−2) over the northern Sahel for all seasons. The reforestation tends to increase the latent heat flux during DJF over the northern Sahel and over the entire Sahel domain during JJA, MAM and SON with strong values (>30 W∙m−2) over the reforested zone during the summer season JJA (
The analysis of the surface net upward longwave flux (
over the reforested area and during the warm period (MAM) over the eastern Sahel. This decrease appears partly as an increase in the sensible heat flux (
between both runs (down). RegCM4 experiment exhibits lower frequency (<6%) of the TG90P indice over the Central Sahel from December to February. The maximum of the TG90P indice (>22%) is recorded over the northern Sahel from June to August, over the eastern Sahel from September to November and over the entire Sahel during the MAM period. Between 20% and 24% of the warm days occurred during JJA over the northern Sahel and during MAM over most part of the Sahel. The difference between the two simulations of the model shows that the reforestation could influence strongly the frequency of the TG90P. This indice decreases over the reforested zone during the warm period (MAM), over the western and eastern Sahel during SON and over the entire Sahel during the summer period (JJA).
We also computed and displayed in
Sahel and the North-East part of the Sahel from September to November (SON) and over the southern Sahel during the summer season (JJA). Overall, the maximum of the TX90P indice (>18%) is recorded over the western Sahel during JJA, over the South-East part of the eastern Sahel during SON and over the entire Sahel during the warm season (MAM). Between 18% and 20% of the very warm days occurred during the warm season (MAM) over the northern Sahel in coherence with [
Two absolute extremes which are relevant for impacts studies over the Sahelian region have been considered: the frequency of very hot days (HtD: days with TX > 35˚C) and the tropical nights (TrN: nights with TN > 20˚C) as considered in [
the atmospheric moisture content (
In order to go deeper in the analysis of thermal extremes, the seasonal variability of heat waves days and warm spell days are displayed respectively in
days of all heat waves events of the study period. When considering the control run, heat waves days mainly occurred over the southern Sahel during JJA and SON periods and over the northern Sahel during the warm season (MAM) (
The frequency of warm spell days (WSD) of daily mean temperature is calculated and displayed in
Atlantic Ocean during the warm season (MAM) and SON period. Shorter values (<3 days) are found especially over the continent during MAM, JJA and SON. The model simulates shorter values (<2 days) of the frequency of WSD over most part of the coastal regions. When considering the difference between the two versions, a decrease (an increase) of the warm spell days frequency is noticed over the entire Sahel domain during the DJF period (respectively over the Central Sahel during the warm season (MAM) and over the southern Sahel during JJA).
In summary, this study shows that the reforested zone could decrease the warm extremes indices values (TG90P, TX90P and TN90P) over some areas of the Sahel especially during the warm season (MAM) and the summer season (JJA). Nevertheless some Sahelian regions like the eastern part of the Central Sahel could suffer from this land cover change because of the increase of thermal extremes indices.
This work performed two sets of regional climate model experiments (with and without reforestation) to evaluate the impacts of the reforestation of the Sahel-Sahara interface on the seasonal distribution of thermal extremes over the Sahel. The main results of the study are summarized in the following lines.
The reforestation may influence strongly the frequency of warm days (TG90P) by decreasing it over the reforested zone during MAM, the western and eastern Sahel during SON and over the entire Sahel during JJA. The analysis shows a decrease of the frequency of very warm days (TX90P) over the reforested zone during MAM (warm season) and the entire Sahel during JJA (summer season). The TG90P and TX90P indices decrease may be due to an increase of the atmospheric moisture content over the entire Sahel for all seasons and a decrease of the sensible heat flux over the reforested zone. The analysis of the frequency of warm nights (TN90P) shows a decrease over the northern Sahel during DJF, the northern and Central Sahel during MAM, the western and eastern Sahel during SON and over the entire Sahel during the wet season (JJA).
When considering the variability of hot days, the impact of the reforestation is to decrease them over the entire Sahel for all seasons especially during JJA. This is partly associated to a decrease of the sensible heat flux over the reforested zone and to the strengthening of the atmospheric moisture content over the entire Sahel.
Tropical nights (TrN) are stronger over the southern Sahel during MAM and SON and JJA. The analysis of the difference between the control and the reforested versions shows an increase of the tropical nights over the entire Sahel during DJF and MAM and over the northern Sahel during SON. This may be associated partly to an increase of the surface net downward shortwave flux over the reforested zone.
In order to go deeper in the analysis of extreme thermal indices, heat waves and warm spells indices were analyzed. The heat waves days mainly occurred over the south-west part of the Sahel during JJA when considering the standard version. The reforestation increases the frequency of these events over South Senegal-North Guinea during SON and between North Senegal-South Mauritania during JJA and decreases them over the entire Sahel region during DJF. When considering the warm spell days, the reforestation tends to decrease them over the entire Sahel domain during the DJF period. Changes in latent heat flux appear to be largely responsible for these extreme temperatures variations.
Finally, the results show that the reforestation decreases the warm extremes indices values over some Sahelian regions especially during the warm (MAM) and the summer seasons (JJA) suggesting that this land cover change could have a positive impact on human health particularly over the reforested area. Nevertheless, some Sahel regions like the eastern part of the central Sahel could suffer from this land cover change because of the increase of thermal extremes values. Moreover a study performed by [
Despite the results obtained in this work, additional works are needed to better explain the impacts of the reforestation on the seasonal variability of surface temperature and thermal extremes over the Sahel for the present day but also for the near (2050) and far (2100) future using Coupled Model Intercomparison Project 5 (CMIP5) climate change projections dynamically downscaled by the RegCM4 model.
The authors would like to thank the Assane SECK University of Ziguinchor, the FIRST program of MESRI-Senegal and the SCAC of the French ambassy in Senegal for their support. This study is a part of ANR project ACASIS (2014-2017, grant: ANR-13-SENV-0007). This work is also partly funded by the NERC/DFID Future Climate for Africa programme under theAMMA-2050 project; grant number NE/M019969/1.
The authors declare no conflicts of interest regarding the publication of this paper.
Diba, I., Camara, M. and Diedhiou, A. (2019) Impacts of the Sahel-Sahara Interface Reforestation on West African Climate: Intra-Annual Variability and Extreme Temperature Events. Atmospheric and Climate Sciences, 9, 35-61. https://doi.org/10.4236/acs.2019.91003